This invention relates to feedthroughs for providing an electrical path for implantable medical devices including electrical pulse generators. Examples of such devices are implantable cardiac pacemakers and implantable cardiac defibrillators for correction of cardiac abnormalities. The pacemaker or defibrillator device has a housing containing a pulse generator including associated circuitry and a battery that serves as a power supply. A conductive lead or pin extends from the pulse generator circuit in the interior of the device and passes through the device housing where it is connected via a medical lead to an electrode surgically attached to an appropriate location in the heart.
One of the concerns related to the use of such implantable medical devices (pacemakers, defibrillators, etc.) is that they are subject to stray electromagnetic interference (EMI). Such EMI may come from sources such as television transmitters, cell phones, theft detection devices and so on. This spurious EMI is highly undesirable because it can interfere with proper functioning of the implanted medical device, either by inhibiting a proper response or by causing an improper one. Such stray EMI can essentially be eliminated as a problem source by shunting the EMI to ground with the use of a filter capacitor connected between the input lead wire(s) and electrical ground. Typically, one capacitor is positioned between each such lead wire and ground. These capacitors are often built into a monolithic structure or array when used for a multilead feedthrough. If the array is in the form of a right circular cylinder, it is designated a discoidal capacitor.
However, these prior art type feedthroughs routinely use conductive polymeric materials such as polyimides and epoxies or metallic materials such as solder alloys for holding their constituent parts together. Use of the conductive polymeric materials requires care in preventing leakage of the conductive polymer into locations in the assembly where it could cause a short circuit rendering the implantable medical device inoperative. In addition, conductive polymers exhibit relatively low electrical conductivity as compared with metallic materials. The bonding mechanism between the conductive polymer and the metallic members of the feedthrough is predominately mechanical, resulting in a relatively weak electrical and mechanical connection. Solders have relatively low melting temperatures such that subsequent high temperature welding operations on other parts of the device can compromise the soldered joint or cause beading in which a ball or pellet of solder could fall into a location in the device where a short circuit could result. Additionally, some soldering operations require the use of fluxes that leave behind undesirable residues after the soldering is completed, that can be a source of entrapped moisture, possibly resulting in device failure. Thus, there is a need for a better filtered feedthrough device as well as a better filtered feedthrough assembly process.
The present invention provides a feedthrough assembly and a method of making the same wherein capacitive arrays are installed into a single or multi-pin feedthroughs using a brazing process. The braze material serves to join the capacitor to the feedthrough, holding it securely in place. In addition, the braze material provides the electrical connection from one set(s) of internal capacitor plates to the flange or ferrule and from the opposing set(s) of plates to the feedthrough lead wire(s).
In particular, the feedthrough comprises a lead or conductive pin, a ferrule defining a capacitor receiving recess and an insulator receiving recess, a capacitor disposed in the capacitor receiving recess and defining a capacitor passageway for the lead to pass therethrough, and an insulator disposed in the insulator receiving recess and defining an insulator passageway for the lead to pass through. The lead or pin passes through the insulator in a nonconductive manner. The capacitor comprises first and second sets of plates separated by a dielectric, the first set of plates being conductively coupled to the ferrule and the second set of plates being conductively coupled to the lead so that the lead or pin passes through the ferrule in a non-contacting and nonconductive manner. An insulator braze material is used for forming the insulator-lead braze joint and the insulator-ferrule braze joint. A capacitor braze material is used for forming the capacitor-ferrule braze joint and the capacitor-lead braze joint.
The insulator braze material is typically gold, while the capacitor braze material is a composition selected to be compatible with the termination materials used in the capacitor. The brazing process is a two step procedure wherein the first step calls for the brazing of the insulator-lead braze joint and the insulator-ferrule braze joint at a first temperature using a selected insulator braze material. The second step of brazing calls for brazing the capacitor-ferrule braze joint and the capacitor-lead braze joint, with the selected capacitor braze material at a second temperature that is lower than the first temperature. The braze materials for each step described in the detailed description. This second brazing process does not damage, weaken, or otherwise destroy the insulator-ferrule braze joint or insulator-lead braze joint formed in the previous operation (first step of the brazing process) because if is performed at a lower temperature.
A durable feedthrough assembly is thus provided that is superior to the prior art because the feedthrough can withstand subsequent welding processes without losing its integrity and because the constituent parts of the feedthrough are brazed together. If a solder were used, it might melt, weaken, and bead up at the increased temperatures encountered during welding and a compromised joint could thus result. Furthermore, beads of solder could form if soldering were employed and they could fall into the region of the implantable device containing the electrical components causing short circuits and other problems. The present invention avoids these problems and thus successfully overcomes problems associated with the prior art.